1. Field of the Invention
This invention relates in general to spread spectrum receivers and in particular to GPS navigation systems, such as those used in terrestrial navigation for cars, trucks and other land vehicles, operated in environments with substantial multipath interference.
2. Description of the Prior Art
Car navigation is conventionally performed using highway and street maps aided, to some degree, by distance measurements from external sensors such as odometers. Improvements over the last 10 years in Global Positioning System, or GPS, satellite navigation receivers has spawned several GPS car navigation systems.
Conventional GPS car navigation systems use the last known position of the vehicle, and the destination data, to compute a route data base, including route and turning data derived from a pre-existing map data base. GPS receivers are conventionally operated with a minimum of 3 or 4 satellites distributed across the visible sky in order to determine, or at least estimate, the four necessary unknowns including x.sub.user, y.sub.user and z.sub.user which provide three orthogonal coordinates to locate the user as well as t.sub.user which provides the required satellite time. Techniques such as time or clock hold and altitude hold, in which the unknown time or altitude is assumed to remain predictable from a previously determined value, e.g. z.sub.est and/or t.sub.est, have permitted operation of GPS receivers with less than 4 satellites in view. In particular, terrestrial GPS receivers have been operated with as few as 2 satellites to provide a 2 dimensional position solution using both clock and altitude hold.
Because continuous reception from 4 GPS satellites is often difficult to maintain in a car navigation environment, and known clock and altitude hold techniques can only permit operation with at least 2 satellites, known conventional car navigation systems have typically augmented the GPS position information with information from external sensors to provide dead reckoning information. The dead reckoning information is often provided by an inertial navigation system such as a gyroscope.
Augmenting GPS data with inertial navigation data has permitted the use of GPS car navigation even when less than 4 satellites are visible, such as in tunnels and in urban situations between tall buildings. However, the resultant increased complexity and costs for such combined systems have limited their acceptance.
Conventional GPS receivers use separate tracking channels for each satellite being tracked. Each tracking channel may be configured from separate hardware components, or by time division multiplexing of the hardware of a single tracking channel, for use with a plurality of satellites. In each tracking channel, the received signals are separately Doppler shifted to compensate for the relative motion of each satellite and then correlated with a locally generated, satellite specific code.
During a mode conventionally called satellite signal acquisition, delayed versions of the locally generated code for the satellite being acquired are correlated with the Doppler rotated received signals to synchronize the locally generated code with the code, as received for that satellite, by determining which delay most accurately correlates with the code being received. Once synchronization has been achieved for a particular satellite, that satellite channel progresses to a tracking mode in which the Doppler rotated, received signal is continuously correlated with the locally generated code for that satellite to determine position information including pseudorange information. During tracking, conventional receivers also correlate the Doppler shifted received signal with one or more versions of the locally generated code at different relative delays, such as one half C/A code chip width early and late relative to the synchronized or prompt version of the code. These early and late correlations are used to accurately maintain the synchronization of prompt correlation.
When, after tracking has begun for a particular satellite, the satellite signal has been lost so that the required timing of the locally generated code for synchronization is no longer accurately known, conventional receivers reenter the acquisition mode, or a limited version of this mode, to reacquire the satellite signals by multiple correlations to resynchronize the locally generated code with the code as received. Once the locally generated code has been resynchronized with the signals as received, position information data is again derived from the signals from that satellite.
GPS systems, as well as many other radio frequency (RF) communication systems utilizing frequencies high enough to be considered line of sight systems in which there must be a substantially direct line of sight between the transmitter(s) and receivers(s) for optimum operation, often suffer from multipath effects in which the receiver(s) must process signals received over a multiplicity of different paths. A common example is a simple broadcast TV system in which a TV receiver with an antenna receives multiple copies of the signal being transmitted.
The multiplicity of signals being received results from additional, typically unwanted, signal paths including one or more reflections. When the signal path from the transmitter to receiver includes a reflection, this signal path must by definition be longer than the direct path. Multipath signals present a problem in systems, such as GPS systems, in which the time of arrival of the signal is to be measured or used because the time of arrival of the multipath signals depends on the length of the path(s) taken.
The straightforward processing of all signals, including multipath or reflected signals, often degrades the processing performed by the receiver. In the simple broadcast TV transmission system described above, the processing of unmodified multipath signals by the receiver results in the commonly experienced degradation called "ghosting" in which multiple signals are displayed offset in the TV image. The multiplicity of displayed offset video signals results from the difference in path lengths of the various multipath signals received.
The direct path is the shortest and therefore requires the least travel time from transmitter to receiver while the various unwanted multipath signals have various greater lengths, and therefore various longer travel times, than the direct path signals. Signals are processed in part in a TV receiver in accordance with their time of arrival and therefore the resultant video display may include a plurality of images slightly displaced in space on the TV monitor in accordance with their different path lengths.
Many conventional partial solutions to the problems of multipath reception exist. In the TV broadcast example, a highly direction antenna is often used for the receiver to reduce the number of multipath signals processed by the receiver. In addition, various discrimination techniques have been developed which use the knowledge that the amplitude of the direct path signal is typically substantially greater than that of the unwanted multipath signals because signal amplitude is degraded by the square of the path length.
In other types of systems, such as the GPS systems using PRN encoded spread spectrum signals, certain conventional techniques are difficult or impossible to use. For example, GPS transmitters are positioned on satellites with complex orbital paths so that the locations of the multiple transmitters are constantly changing. This makes a highly directional antenna system almost completely unusable. Similarly, digital receivers, including those used in a GPS receiver, often do not rely solely on the amplitudes of the signals received, but rather rely on other signal characteristics, such as time of arrival.
Multipath processing techniques currently used for complex receivers, such as GPS receivers, are often quite complex and subject to inaccuracies. An example of one such conventional technique is described in U.S. Pat. No. 5,414,729 issued on May 9, 1995 to Patrick Fenton and assigned as issued to NovAtel Communications Ltd., Canada. In this technique, an autocorrelation function of a partially processed received signal, including multipath components, is compared to an estimated autocorrelation function of an estimated direct path signal to attempt to discern direct path signals from multipath signals for further processing. This technique of comparing processed and estimated correlation power, is complex and may be subject to error in that the partially processed signals relied on are themselves subject to degradation from many effects in addition to multipath effects including receiver limitations, which may reduce the accuracy or effectiveness of the multipath processing techniques.
For example, in tracking a GPS C/A signal to determine position information from GPS satellite transmitters, it is typically important to derive an accurate estimate of the time of arrival, known as code phase, of the PRN modulation of the direct path component of the C/A signals received from each of the various GPS satellites. It is also important to derive an accurate estimate of the phase of the underlying carrier signals transmitted from the satellites on which the modulation is applied, known as the carrier phase. However, as apparently shown for example in FIGS. 6, 7 and 8 of the above referenced Fenton patent, the delayed multipath components degrade the tracking of the code and carrier phase estimates by distorting the correlation functions used is such tracking.
What is needed is an improved spread spectrum receiver, such as one for use with GPS navigation systems, which avoids the limitations of conventional designs and provides improved results in a wide range of reception conditions, including multipath interference.